1. Field of the Invention
The present invention relates to an improved process for the production of polyisocyanates which have a biuret structure, excellent color quality and good monomer stability, by reacting aliphatic diisocyanates with water in the presence of specific carboxylic acids and/or the anhydrides thereof as catalysts.
2. Description of the Prior Art
Aliphatic polyisocyanates which have a biuret structure and in particular which are based on hexamethylene diisocyanate have achieved world-wide commercial importance for the production of light-fast lacquers which are extremely resistant to the effects of exposure and have as good a gloss-life as possible. Market demand for use in this field and in particular for clear and white-pigmented coatings, is for slightly colored or colorless products. Moreover, as small as possible a quantity of monomeric diisocyanates is required for safe processing, a quantity which does not increase even after a relatively long period of storage. Based on toxicological tests, safe processing is possible up to a maximum content of 0.7% of monomeric diisocyanate, as long as standard safety regulations are adhered to when processing the lacquer. The above-mentioned ceiling is found in the literature (e.g. Code of Practice "PUR-Paints" by the main association of the German commercial professional body and the "Polyurethane Report" by the Paintmakers' Association).
Over the years, a large number of processes have become known for producing polyisocyanates of this type, each of which suffers from specific problems and disadvantages and does not meet or meets only unsatisfactorily the above-mentioned demands on the product. The following processes are described by way of example:
Synthesis from diisocyanates and water, optionally in the presence of catalyst; c.f. DE-PS No. 1 110 394, DE-OS No. 1 668 377, DE-OS No. 2 308 015, GB-PS No. 889 050, GB-PS No. 1 399 228, DDR-PS No. 140 744.
Synthesis from diisocyanates and water in the presence of a solvent or a solvent mixture; c.f. DE-OS No. 2 808 801, DE-OS No. 3 030 655.
Synthesis from diisocyanates and water, the water being reacted in the form of vapor; c.f. DE-OS No. 2 918 739.
Synthesis from diisocyanates and hydrogen sulphide, optionally in the presence of catalyst; c.f. DE-AS No. 1 165 850.
Synthesis from diisocyanates and ammonia or ammonia-water mixtures, optionally in the presence of catalyst; c.f. DE-AS No. 1 227 003.
Synthesis from diisocyanates and amines: c.f. DE-PS No. 1 165 850, DE-PS No. 1 174 759, DE-OS No. 1 568 017, DE-OS No. 1 693 190, DE-OS No. 2 010 887, DE-OS No. 2 261 065, DE-AS No. 2 438 258, U.S. Pat. No. 3,824,266, DE-AS No. 2 609 995, DE-OS No. 2 803 103, DE-PS No. 883 504, GB-PS No. 1 263 609, c.f. also Angew. Chem. 72 P 1002.
Synthesis from diisocyanates and amine/alcohol mixtures; c.f. DE-OS No. 2 654 745.
Synthesis from diisocyanates and .omega.,.omega.'-diaminopolyethers; c.f. DE-OS No. 1 570 632, DE-AS No. 1 215 365.
Synthesis from diisocyanates and substituted ureas; c.f. DE-PS No. 1 101 394, DE-AS No. 1 227 004
Synthesis from diisocyanates and tertiary alcohols, optionally in the presence of catalysts; c.f. DE-AS No. 1 543 178, DE-AS No. 1 931 055, DE-OS No. 2 308 015.
Synthesis from diisocyanates and formic acid; c.f. DE-PS No. 1 174 760, DE-OS No. 2 308 015, DE-OS No. 2 437 130.
Synthesis from diisocyanates and aldoximes; c.f. DE-OS No. 3 007 679.
The known processes in which the diisocyanates are directly reacted with water are difficult to control on account of the non-homogeneity of the reaction mixture. This can lead to the formation of polyurea which dissolves with extreme difficulty. The dissolution of the polyurea necessitates the use of elevated temperatures over a long period of time, thereby impairing the color of the product. Even then, under certain circumstances, some of this polyurea remains undissolved as a precipitate which can only be filtered with difficulty and, thus, has to be separated by a costly operation before further processing. Furthermore, on account of the volatility of most diisocyanates, deposits of urea may form in the steam chamber of the reaction vessel. This is also the case in processes in which water is used in vapor form.
These deposits can only be avoided in hitherto known processes which use water as a biuretization agent if solvents or solvent mixtures are used to homogenize the reaction mixture. These processes, however, suffer from various disadvantages. On the one hand, large quantities of solvent are necessary which then have to be removed by distillation in a later stage of the process to produce the finished product and on the other hand, specific solvent mixtures of glycol ether acetates and phosphoric acid esters are required for colorless products. Furthermore, a reaction temperature of at least 140.degree. C. is necessary in these processes to prevent the intermediate precipitation of insoluble ureas. If precipitates of this type are nevertheless formed, for example as a result of relatively low reaction temperatures, as in the processes in which water is used in the absence of solvent, a temperature of 160.degree. C. and above is necessary to produce a clear product. This thereby leads to a more frequent occurrence of by-products and to a marked deterioration in the color quality.
Processes may be carried out in the absence of solvent by releasing water from a water-splitting compound during the reaction. The processes include in particular the commercially important process which uses tert.butanol and other tert. alcohols as biuretization agents. This process, however, also requires a temperature of about 180.degree. C. involving all the above-mentioned disadvantages regarding the quality of the product. Furthermore, this process involves the loss of the biuretization agent with the release of combustible gases (isobutene).
The reaction of diisocyanates with aldoximes is also characterized by the loss of the biuretization agent which is difficult to obtain and the production of easily-volatile by-products (nitriles) which cannot be re-used.
Reacting diisocyanates with hydrogen sulphide produces the toxic, low-boiling carbon oxysulphide which cannot be re-introduced into the process and has to be removed by a costly operation.
All the above-mentioned processes share the common factor that some of the diisocyanate is converted into amines, that is the precursor of the isocyanates, by reaction with the biuretization agent. For this reason, processes were proposed in which diisocyanates are reacted directly with the diamine-precursors thereof to produce the biuret-polyisocyanates. However, in paricular in the case of the most important substituent from a commercial point of view, 1,6-diisocyanatohexane, and even when highly-developed mixing processes are used, polyureas are produced which only dissolve with difficulty on account of the high reactivity of the diamines. The dissolution of the polyureas requires extremely high temperatures and this is accompanied by the deterioration in the color quality and an increased frequency of by-products. Carbodiimides and the secondary products of carbodiimides are produced in addition to dimeric uretdiones and trimeric isocyanurates. The carbodiimides have an adverse effect on the monomer stability of the end product.
The tendency to form polyureas which only dissolve with difficulty may be reduced by using diamines whose carbon frame does not correspond to the diisocyanates which are used and whose reactivity may be markedly reduced in a suitable manner, for example by steric hindrance. The products contain, among other things, a large quantity of monomeric diisocyanates which are produced from the diamine reactants and which cannot be reduced by thin-layer distillation.
If .omega.,.omega.'-diaminopolyethers are used, liquid, biuret-containing polyisocyanates are obtained; this solution is, however, costly, on account of the additional synthesis of the biuretization agent. Moreover, the ether groups which are present in these products cause the lacquer products which are produced therefrom to have poor behavior under the effects of exposure.
The formation of polyureas may be prevented by using monoamines or N,N'-disubstituted ureas. In this case, however, the volatile monoisocyanates which result from these biuretization agents have to be removed from the reaction mixture. This is only partially possible, even at elevated temperature, because of the chemical equilibriums of the reactions which take place.
Products which have good color quality may be produced by carefully reacting diisocyanates with formic acid, but these products still contain a large quantity of N-formyl groups. To produce a polyisocyanate with an essentially biuret structure, a reaction temperature of greater than 160.degree. C. is required over a relatively long period of time and this produces a marked yellowing of the products. The biuretization agent is moreover consumed with the release of toxic carbon monoxide, thereby causing considerable problems when compared to processes which were also proposed in which ammonia and amine-alcohol mixtures respectively are used. Apart from other disadvantages, processes of this latter type produce products which have a modified structure and a different property spectrum. This holds true for products which are produced by reacting diisocyanates with ammonia.
It has now been found that polyisocyanates which have a biuret structure, outstanding color quality and good monomer stability may be produced by reacting an excess of aliphatic diisocyanates with water in the presence of specific tri-substituted acetic acids or anhydrides of the said carboxylic acids.